TiO2 films or coatings are useful in a number of applications. For example, a surface coated with a photo-catalytic TiO2 film can be self-cleaning. When the surface is exposed to, for example, ultra-violet (UV) light, absorption of the light can induce oxidation-reduction reactions on the surface thus producing a cleaning effect. It is known that TiO2 can be amorphous or in one of three possible crystalline phases, anatase, brookite and rutile. For self-cleaning purposes, anatase TiO2 exhibits more stable photo-catalytic activity than the amorphous and the other two crystalline phases.
Amorphous TiO2 can be converted to anatase TiO2 at temperatures around 500° C. Thus, in many conventional techniques, anatase TiO2 particles are formed by first forming amorphous TiO2 particles and then sintering or calcining the amorphous TiO2 particles (can be in a film) at a high temperature, such as at around 400 to 500° C., to obtain anatase TiO2. A drawback of such techniques is the necessity to heat the particles or film to high temperatures. Heating to high temperatures may not be desirable, or even possible, in many applications. For example, when heated, thermal-mismatch can cause de-lamination of the coated TiO2 film from the underlying substrate. It would be impossible to cure TiO2films on glass windows already installed on a building. Further, some substrates, such as textile or polymer substrates, may have low temperature resistance and thus it would be difficult to calcine TiO2 films or powder coatings on them.
There are also known techniques for forming anatase TiO2 films without sintering or calcination. For example, anatase TiO2 can be formed by a gel-sol method described in Tadao Sugimoto et al., “Synthesis of uniform anatase TiO2 nanoparticles by gel-sol method. 3. Formation process and size control”, Journal of Colloid and Interface Science, (2003) vol. 259, pp. 43-52, the contents of which are incorporated herein by reference. In this approach, TiO2 nanoparticles were prepared by aging a solution containing Ti4+ ions and triethanolamine (TEOA). In a first aging stage, the solution was aged at 100° C. for 24 hours to form a Ti(OH)4 gel. In a second aging stage, the gel was aged at 140° C. for 72 hours to nucleate and grow anatase TiO2 particles. It is reported that almost all TEOA molecules were liberated from the Ti(IV) ions after the first aging stage. However, this approach has some problems. For example, when the formed TiO2 nanoparticles are used to form photo-catalytic films, the photo-catalytic activities of the films are not very high. The films also tend to crack.
Accordingly, there is a need for an improved or alternative approach of forming anatase TiO2. There is also a need for a photo-catalytic TiO2 film with improved properties.